1. Field of the Invention
This invention relates to alloy steels for use as suspension springs for vehicles. More particularly, it relates to compositions of Spring steels wires.
2. Description of the Prior Art
For the spring steel wires of the just-mentioned type, there have been heretofore mainly used steels of JIS SUP 6-7, but from a viewpoint of energy savings, it is required to make the vehicles lighter in weight. In addition, an expressway network has been fully developed with an increasing chance of travelling at high speeds. In the winter season of cold districts, it is often experienced to spray salt as one of measures against deep snows on road, so that suspension springs which are mounted in an environment completely different from valve springs incorporated in engine becomes susceptible to corrosion fatigue or delayed fracture. Accordingly, suspension springs which are one of important safety parts are required not only to have the performance as spring, but also to be reliable. Under these circumstances, hitherto employed spring steel wires are found to be disadvantageous in that they show a great settling or permanent set in fatigue and poor in resistance to corrosion fatigue or delayed fracture and that when they are imparted with high strenghs by heat treatment in order to improve the settling resistance, the reliability in performance is lost due to the lack of toughness.
It will be noted that the steel wires of the type to which the present invention is directed are known, for example, in Swedish Pat. No. 342,475 (corresponding to British Pat. No. 1,300,210), U.S. Pat. Nos. 1,972,524, 2,395,687 and 3,528,088, Japanese Patent Publication No. 46-19420 and Russian Standards 60C 2 x A and 60C 2 x .PHI.A.